systems analysis and design tools data flow diagrams

SYSTEMS ANALYSIS AND DESIGN TOOLS

DATA FLOW DIAGRAMS

INTRODUCTION

Data Flow diagrams graphically characterize data processes and flows in a business system.

In their original state data flow diagrams depict the broadest possible overview of system inputs, processes and outputs, which correspond to those of the general systems model discussed.

A series of layered data flow diagrams may also be used to represent and analyze detailed procedures within a larger system.

THE DATA FLOW APPROACH

When systems analysts attempt to understand he information requirements of users, they must be able to conceptualize how data moves through the organization, the processes or transformation that the data undergoes what the outputs are

The data flow approach emphasizes the logic underlying the system.

By using combinations of only four symbols, the systems analyst can create a pictorial depiction of processes that will eventually provide solid system foundation.

ADVANTAGES OF USING DFD

1. Freedom from committing to the technical implementation of the system too early.

2. Further understanding of the interrelatedness of systems and subsystems.

3. Communicating current system knowledge to users through data flow diagrams.

4. Analysis of a proposed system to determine if the necessary data and processes have been defined.

CONVECTIONS USED IN DFD

Four basic symbols are used to chart movement on data flow diagrams. They are: A Double Square An arrow A rectangle with rounded corners A rectangle

An entire system and sub-system can be depicted graphically with these four symbols in combination.


Entity StudentStudent

Flow of data

New Student Information

Process2.1

Create Student record

2.1

Create Student record

Data Store Student MasterStudent MasterD3

4 BASIC SYMBOLS USED IN DFD


THE DOUBLE SQUARE

This is used to depict an external entity (another department, a business, a person or a machine) that can send data to or receive data from the system.

The external entity or entity, is also called a source or destination of data, and it is considered to be external to the system being described.

Each entity is labeled with an appropriate name.


THE DOUBLE SQUARE

Although it acts with the system it is considered as outside of the boundaries of the system.

Entities should be named with a noun.

The same entity may be used more than once on the same data flow diagram, to avoid crossing data flow line.


THE ARROW The arrow shows movements of data from one point to another, with the head of the arrow

pointing towards the data’s destination.

Data flows occurring simultaneously can be depicted doing just that through the use of parallel arrows.

Since an arrow represents data about a person, place or thing, it should be described with a noun.


THE RECTANGLE WITH ROUNDED CORNERS

This is used to show the occurrence of a transformation process.

Processes always denote a change in or transformation of data; hence, the data leaving a process is always labeled differently from the one entering it.

Processes represents work being performed within the system and should be named using one of the following formats.



A clear name makes it easier to understand what the process is accomplishing.

1. Assign the name of the whole system when naming a high-level process. Eg. INVENTORY CONTROL SYSTEM.

2. The name of a sub-system, use a name such as INVENTORY REPORTING SUBSYTEM.

3. Use a verb-adjective-noun format for detailed processes.1. The verb described the activity, for example, COMPUTE, VERIFY,

PREPARE, ADD, or PRINT.2. The noun indicates what the major outcome of the process is, for

example, REPORT or RECORD.



1. The adjective illustrates which specific output, such as, BACKORDERED or INVENTORY.

3. Examples of complete process names are: Compute Sales Tax Verify Customer Account Status Prepare Shipping Invoice Print Backordered Reports Add Inventory Records


THE RECTANGLE

These symbols are drawn only wide enough to allow identifying lettering in the rectangle.

In logical data flow diagrams, the type of physical storage (e.g., tape, diskette, cd) is not specified.

At this point the data store symbol is simply showing a depository that allows addition and retrieval of data

The data store may represent a manual store, such as a filing cabinet, or a computerized file or database.


THE RECTANGLE

Since data store represent a person, place, or thing they are named with a noun.

Temporary data stores, such as scratch paper or a temporary computer file, are not included on the DFD.

Give each data store a unique reference number, such as, D1, D2, D3, and so on, to identify it level.

DEVELOPING DFDDEVELOPING DFDUsing A Top-Down ApproachUsing A Top-Down Approach

1. Make a list of business activities and use it to determine various– External entities– Data flow diagrams– Processes– Data stores

2. Create a Context Diagram which shows external entities and data flow to and from the system. DO NOT SHOW ANY DETAILED PROCESSES OR DATA STORE.


3. Draw Diagram 0, the next level. Show processes, but keep them general. Show data stores at this level.

4. Create a child diagram for each of the processes in Diagram 0.

5. Check for errors and make sure the labels you assign to each process and data flow are meaningful.


6. Develop a physical data flow diagram from the logical data flow diagram. Distinguish between

• manual and automated processes, • describe actual files and reports by name, • add controls to indicate when processes are complete or

error occur.

7. Partition the physical data flow by separating or grouping parts of the diagram in order to facilitate programming and implementation.

RULES FOR DRAWING DATA RULES FOR DRAWING DATA FLOW DIAGRAMSFLOW DIAGRAMS

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11Data Flow should not split into two or more different data flows.

Data Flow should not split into two or more different data flows.


All data flows must EITHER originate or terminate at a process

All data flows must EITHER originate or terminate at a process


Processes needs to have at least one input data flow and one output data flow

Processes needs to have at least one input data flow and one output data flow

CREATING THE CONTEXT CREATING THE CONTEXT DIAGRAMDIAGRAM

• This is an overview, one including basic inputs, The general system and outputs.

• This will be the most general diagram – really, a bird’s-eye view of data movement in the system and the broadest possible conceptualization of the system.

• The context diagram is the highest level in a dat flow diagram and contains only one process, representing the entire system.


• The process is given the number 0.

• All external entities are shown on the contest diagram, as well as major data flow to and from them.

• The diagram does not contain any data stores and is fairly simple to create, once the external entities and the data flow to and from them are known to analysts from interviews with users and as a result to document analysis.


EXAMPLE

Entity 1

Entity 1

Entity 2

Entity 2

Entity 3

Entity 3

0

System Name

0

System Name

Input AInput A

Input BInput B

Output COutput C

systems analysis and design tools data flow diagrams

Documents

data processes

transformation of data

data flows

destination of data

data undergoeswhat

movements of data

data moves

necessary data